T-Mobile? I have a blackberry under a tmobile contract, but i've been researching googles mytouch 3G which is a better fit for me and my business. i found a mytouch on criagslist, but then began to wonder how that would work with my exsising plan with T-mobile. Any thoughts?

Thanks for the quick feedback...ok, so I get the SIM card part, but how do I let T-mobile know that I've changed phones so that I can switch from my blackberry data plan to the mytouch data plan?